In many electronic applications, electrical resonators are used. For example, in many wireless communications devices, radio frequency (RF) and microwave frequency resonators are used as filters to improve reception and transmission of signals. Filters typically include inductors and capacitors, and more recently resonators.
As will be appreciated, it is desirable to reduce the size of components of electronic devices. Many known filter technologies present a barrier to overall system miniaturization. With the need to reduce component size, a class of resonators based on the piezoelectric effect has emerged. In piezoelectric-based resonators, acoustic resonant modes are generated in the piezoelectric material. These acoustic waves are converted into electrical waves for use in electrical applications.
One type of acoustic resonator is a Film Bulk Acoustic Resonator (FBAR). The FBAR has the advantage of small size and lends itself to Integrated Circuit (IC) manufacturing tools and techniques. The FBAR includes an acoustic stack comprising, inter alia, a layer of piezoelectric material disposed between two electrodes. Acoustic waves achieve resonance across the acoustic stack, with the resonant frequency of the waves being determined by the materials in the acoustic stack.
FBARs are similar in principle to bulk acoustic resonators such as quartz, but are scaled down to resonate at GHz frequencies. Because the FBARs have thicknesses on the order of microns and length and width dimensions of hundreds of microns, FBARs beneficially provide a comparatively compact alternative to known resonators.
FBARs may comprise a membrane (also referred to as the acoustic stack) disposed over air. Often, such a structure comprises the membrane suspended over a cavity provided in a substrate over which the membrane is suspended. Alternatively, FBARs comprise the membrane formed over an acoustic mirror formed in the substrate. Regardless of whether the membrane is formed over air or over an acoustic mirror, the membrane comprises a piezoelectric layer disposed over a first electrode, and a second electrode disposed over the piezoelectric layer.
Desirably, the FBAR excites only thickness-extensional (TE) modes, which are longitudinal mechanical waves having propagation (k) vectors in the direction of propagation. The TE modes desirably travel in the direction of the thickness (e.g., z-direction) of the piezoelectric layer. Unfortunately, besides the desired TIE modes there are lateral modes, known as Rayleigh-Lamb modes, generated in the acoustic stack as well. The Rayleigh-Lamb modes are mechanical waves having k-vectors that are perpendicular to the direction of TE modes, the desired modes of operation. These lateral modes travel in the areal dimensions (x, y directions of the present example) of the piezoelectric material. Among other adverse effects, lateral modes deleteriously impact the quality (Q) factor of an FBAR device. In particular, the energy of Rayleigh-Lamb modes is lost at the interfaces of the FBAR device. As will be appreciated, this loss of energy to spurious modes is a loss in energy of desired longitudinal modes, and ultimately a degradation of the Q-factor.
FBARs comprise an active area comprising the region of overlap of the first and second electrodes, and the piezoelectric layer of the membrane, or acoustic stack. The desired mechanical properties and electrical characteristics of the membrane are dependent on the materials used to make up the membrane and their thicknesses. For example, the careful selection of the resonant frequency of the membrane is determined by the careful selection of the materials of the acoustic stack. As should be appreciated, in implementations of the FBAR such as electrical filters, the precision of the resonant frequency of the membrane is important to realize acceptable filtering.
Unfortunately, fabrication of the FBAR devices and exposure to the ambient after fabrication in operational use can result in changes in the materials, and therefore changes in the desired properties and characteristics of the FBAR. For example, exposure to moisture and air can result in the formation of undesired oxides on the membrane. The degree of formation of the oxides cannot be predicted with accuracy, and therefore cannot be accounted for during fabrication of the FBAR. These oxides can alter mechanical and electrical characteristics of the FBAR, such as the resonant frequency, increase electrical losses, and decrease the quality (Q) factor.
One approach to prevent exposure of the FBAR device includes hermetically packaging the acoustic stack before certain processing sequences are undertaken during fabrication. The acoustic stack is thereby protected from exposure of certain chemicals during fabrication and the FBARs are protected from exposure to the ambient after fabrication is completed. While hermetic packaging provides protection of the FBAR device, known hermetic packaging methods and structures add complexity to the manufacturing process and size to the final packaged FBAR device. As such, known hermetic packaging methods and structures can increase the overall size of the packaged FBAR device and the cost of the final packaged FBAR device. With the need for reduced size and cost of electronic components, known hermetic packaging methods present shortcomings.
What is needed, therefore, is an FBAR device and method of fabrication that overcomes at least the shortcomings described above.